Add missing fields to invoice line
Add missing fields to invoice line

The documentation for the invoice object
https://stripe.com/docs/api#invoice_object
shows that a line may have a subscription (string) and a discountable (bool).

I named the subscription Sub to match the equivalent field on other types in
this package.

dominicphillips/stripe-go,froatsnook/stripe-go,hyperworks/stripe-go,shazow/stripe-go,froatsnook/stripe-go,stripe/stripe-go,BearchInc/stripe-go,dominicphillips/stripe-go,mondough/stripe-go,stripe/stripe-go,collinglass/stripe-go,mondough/stripe-go,ChannelMeter/stripe-go,ChannelMeter/stripe-go